Most of the image projection apparatus for producing colored images on a screen either only comprise one image panel: In order to form a colored image the panel needs to be sequentially illuminated with light beams of different colors. If the sequence is displayed fast enough the human eye is not able to timely resolve the resulting color image sequence. As a result the impression of a colored image develops. One method to realize the required sequential illumination is to place dichroic filter segments secured on a rotatable hub in the path of a light beam. Since the rotation of such a color wheel needs to be quite fast, strong centrifugal forces act on the dichroic filters and the means for securing them to the rotatable hub need to provide sufficient stability.
In most cases it is sufficient to use an adhesive only to attach the color filter segments to the hub. U.S. Pat. No. 5,868,482 (Edlinger) discloses such a color wheel comprising a disc shaped carrier and color filter segments attached to the periphery of the disc shaped carrier using an epoxy adhesive. Unfortunately the disclosure tells nothing about the alignment of these color filter segments to the disc shaped carrier. Alignment is crucial because due to the fast rotation the wheel needs to be well balanced and if the segments are not well aligned to the disc shaped carrier additional means for balancing are required. Each step of the assembly comprising applying adhesive, aligning the segments, curing the adhesive and balancing the wheel takes time and hardly allows for standardized or automated procedure.
In addition in some cases it seems to be not sufficient to secure the segments by adhesive only. Sometimes in cases for which, based on force calculations, the “adhesive only” assembly seems to be not sufficient additional mechanical securing means are required. In such cases according to prior art, holes in the filter segments and screws are used to additionally secure the segments to the disc shaped carriers. Unfortunately it is a complicated procedure and the procedure to drill holes into glass and secure the segments with screws results in decreased durability of the segments.
In U.S. Pat. No. 6,011,662 a color wheel is provided that has changeable filter elements. Here a hub has specific filter holding structures, including a spring element along the periphery of the hub. Each color element is mounted on a platform component. The platform component is shaped with concave edge elements that engage corresponding pegs on the hub and a downwardly extending nub that engages with a corresponding cavity in the hub. The platform component has a beveled insertion edge on the top side. The hub has a plurality of pegs and nub cavities. The color element is inserted into the hub and pressed down. The concave edge elements engage the pegs. The nub is inserted into the nub cavity. The color element is held in position on the hub by those structures as long as the bottom surface of the color element is pressed down against the hub. The spring element extends just past the beveled edge on the platform component of the color element. The color element can be lifted from the hub by first lifting the spring element.
The U.S. Pat. No. 6,011,662 is intended to provide a color wheel with easily changeable, user-selected dichroic filters. As can be seen from the description above the system is rather complicated: Filter segments need to be secured to platforms, which comprise springs etc. Therefore this is a rather expensive solution and one skilled in the art would, in his intention to realize a simple and cheap assembly, avoid this way.